It is known that purified bacterial capsular polymers (CP) generally are immunogenic in mature humans but not in young humans or in many animals. Where they are immunogenic, they can be used as vaccines against the corresponding systemic infections. As used in this application, the term "capsular polymers" refers to sugar-containing polymers, such as polymers of sugars, sugar acids, amino sugars, polyhydric alcohols and sugar phosphates, and does not refer to amino acid-containing polymers. These "capsular polymers" are frequently referred to in the medical literature as "capsular polysaccharides", though they may contain linkages other than glycosidic linkages and constituents other than sugars such as those listed above.
The capsular polymers of different bacteria vary widely in immunogenicity in the first year of human life and in animals. Some are moderately active, such as Streptococcus pneumoniae serotype 3 and Neisseria meningitidis serogroup A. The susceptibility to systemic infection by encapsulated bacteria is greater in the first year of life. The immunogenic response to many bacterial capsular polymers in children is age dependent, i.e., immunocompetence to CP increases to adult levels by about six years of age.
Among the inactive CP are those of Haemophilus influenzae type b, Streptococcus pneumoniae serotypes 6 and 12, and Neisseria meningitidis serogroup C. Examples of CP's which give an intermediate response in infants are Streptococcus pneumoniae serotypes 19 and 51.